In els-cas-templates v2.1, the documentation says that biblatex plus biber is recommended. However, if we actually try to use biblatex, the compilation fails.

This is because cas-sc and cas-dc use a length variable that is defined neither in biblatex nor in cas-sc/cas-dc. That length variable is bibsep. It is defined by natbib, which is incompatible with biblatex.

The solution is simple: just define bibsep in the document preamble:

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\makeatletter
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\newlength{\bibsep}{\@listi \global\bibsep\itemsep \global\advance\bibsep by\parsep}
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\makeatother

Then we should be able to discard natbib and use biblatex.